Sulfur is an objectionable element which is nearly ubiquitous in fossil fuels, where it occurs both as inorganic (e.g., pyritic) sulfur and as organic sulfur (e.g., a sulfur atom or moiety present in a wide variety of hydrocarbon molecules, including for example, mercaptans, disulfides, sulfones, thiols, thioethers, thiophenes, and other more complex forms). Organic sulfur can account for close to 100% of the total sulfur content of petroleum liquids, such as crude oil and many petroleum distillate fractions. Crude oils can typically range from close to about 5 wt % down to about 0.1 wt % organic sulfur. Those obtained from the Persian Gulf area and from Venezuela (Cerro Negro) can be particularly high in organic sulfur content. Monticello, D. J. and J. J. Kilbane, "Practical Considerations in Biodesulfurization of Petroleum",IGT's 3rd Intl Symp. on Gas, Oil, Coal, and Env. Biotech., (Dec.3-5, 1990) New Orleans, La., and Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389.
The presence of sulfur in fossil fuels has been correlated with the corrosion of pipeline, pumping, and refining equipment, and with premature breakdown of combustion engines. Sulfur also contaminates or poisons many catalysts which are used in the refining and combustion of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products such as sulfur dioxide leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as on agricultural areas located downwind of combustion facilities. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389. To combat these problems, several methods for desulfurizing fossil fuels, either prior to or immediately after combustion, have been developed.
One technique which is employed for pre-combustion sulfur removal is hydrodesulfurization (HDS). This technique is suitable for the desulfurization of fluid fossil fuels wherein sulfur is present in predominantly organic, rather than pyritic, form. HDS is thus useful for treating petroleum distillate fractions or refining intermediates, liquid motor fuels, and the like. The HDS process involves reacting the sulfur-containing fossil fuel with hydrogen gas in the presence of a catalyst, usually cobalt- or molybdenum-aluminum oxide or a combination thereof, at elevated temperature and pressure. HDS is more particularly described in Shih, S.S. et al., "Deep Desulfurization of Distillate Components", Abstract No. 264B AIChE Chicago Annual Meeting, presented Nov.12, 1990(complete text available upon request from the American Institute of Chemical Engineers; hereinafter Shih et al.), Gary, J. H. and G. E. Handwerk, (1975) Petroleum Refining: Technology and Economics, Marcel Dekker, Inc., New York, pp. 114-120, and Speight, J. G., (1981) The Desulfurization of Heavy Oils and Residue, Marcel Dekker, Inc., New York, pp. 119-127. HDS is based on the reductive conversion of organic sulfur into hydrogen sulfide (H.sub.2 S), a corrosive gaseous product which is removed from the fossil fuel by stripping. Elevated or persistent levels of hydrogen sulfide are known to inactivate or poison the chemical HDS catalyst, complicating the desulfurization of high-sulfur fossil fuels.
It is also known that the efficacy of HDS treatment for particular types of fossil fuels and refining fractions varies due to the wide chemical diversity of hydrocarbon molecules which can contain sulfur atoms or moieties. Some classes of organic sulfur molecules are labile and can be readily desulfurized by HDS; other classes are refractory and resist desulfurization by HDS treatment. The classes of organic molecules which are often labile to HDS treatment include mercaptans, thioethers, and disulfides. Conversely, the aromatic sulfur-bearing heterocycles (i.e., aromatic molecules bearing one or more sulfur atoms in the aromatic ring structure itself) are the major class of HDS-refractory organic sulfur-containing molecules. Typically, the HDS-mediated desulfurization of these refractory molecules proceeds only at temperatures and pressures so extreme that valuable hydrocarbons in the fossil fuel or refining fraction can begin to deteriorate. Shih et al.
Recognizing these and other shortcomings of HDS, many investigators have pursued the development of commercially viable techniques of microbial desulfurization (MDS). MDS is generally described as the harnessing of metabolic processes of suitable bacteria to the desulfurization of fossil fuels. Thus, MDS typically involves mild (e.g., ambient or physiological) conditions, and does not involve the extremes of temperature and pressure required for HDS. It is also generally considered advantageous that biological desulfurizing agents can renew or replenish themselves under suitable conditions.
The discovery that certain species of chemolithotrophic bacteria, most notably Thiobacillus ferrooxidans, can obtain energy for metabolic processes from the oxidation of pyritic (inorganic) sulfur into water-soluble sulfate has spurred efforts to develop an MDS technique suitable for desulfurizing coal, a fossil fuel in which pyritic sulfur is known to generally predominate. For example, Detz, C. M. and G. Barvinchak U.S. Pat. No. 4,206,288 (issued 1980) describe an aerobic fermentation method for the microbial desulfurization of a coal slurry based upon the metabolic properties of actively growing T. ferrooxidans organisms. Recently, Madgavkar, A. M. U.S. Pat. No. 4,861,723 (issued 1989), has proposed a continuous T. ferrooxidans-based MDS method for desulfurizing particulate coal and preparing a clean burning desulfurized coal-water admixture. Despite this progress, a commercially viable MDS process for desulfurizing coal has not yet emerged, due in part to the time (days to weeks) required for the desulfurizing fermentation step.
As noted previously, T. ferrooxidans-mediated MDS techniques are restricted to the treatment of fossil fuels in which inorganic sulfur, rather than organic sulfur, predominates. Progress in the development of an MDS technique appropriate for the desulfurization of fossil fuels in which organic sulfur predominates has not been as promising. Several species of bacteria have been reported to be capable of catabolizing (metabolically breaking down) sulfur-containing hydrocarbons into water-soluble sulfur products. One early report in this field describes a cyclic catabolic MDS process employing cultures of Thiobacillus thiooxidans, Thiophyso volutans, or Thiobacillus thioparus as the microbial agent. Kirshenbaum, I., U.S. Pat. No. 2,975,103 (issued 1961). Subsequently, Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389, and Hartdegan, F. J. et al., (May 1984) Chem. Eng. Progress 63-67, reported that such MDS processes are, for the most part, merely incident to the metabolic consumption of the hydrocarbon matrix by the microorganisms, rather than sulfur-selective or sulfur-specific phenomena. Moreover, catabolic MDS proceeds most readily on the classes of organic sulfur molecules described above as labile to HDS.
Although Monticello and Finnerty report that several species of bacteria, in particular Pseudomonas putida and P. alcaligenes, have been described as capable of desulfurizing HDS-refractory aromatic sulfur-bearing heterocycles, this reactivity is also merely incident to the consumption of these molecules as a carbon source. Consequently, in catabolic MDS, valuable combustible hydrocarbons are lost. Monticello and Finnerty additionally point out that the water-soluble sulfur products generated from the catabolic MDS of sulfur-bearing heterocycles are small organic molecules rather than inorganic sulfur ions. In view of these findings, the authors conclude that the commercial viability of MDS technology is limited. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389.
None of the above-described desulfurization technologies provides a commercially viable means for liberating sulfur from refractory organic molecules, such as the sulfur-bearing heterocycles, without a concomitant unacceptable deterioration of the fuel value of the treated (desulfurized) product. The interests of those actively engaged in the refining and manufacturing of petroleum fuel products have accordingly become focused on the need to identify such a desulfurization method. This need is driven in part by the prevalence of HDS refractory molecules in crude oils derived from such diverse locations as the Middle East (wherein about 40% of the total organic sulfur content is present in aromatic sulfur-bearing heterocycles) and West Texas (where such molecules account for up to about 70% of the total sulfur), and in part by the increasing stringency of environmental regulations pertaining to the combustion of sulfur-containing fossil fuels.